It is well known to form synthetic polymers into lengths of material by passing the polymer, whilst in the soft or molten state, through an orifice in a die. Such a process is known as extrusion moulding and can be applied to a wide range of materials, for example materials which may undergo a physical, chemical or crystallographic change from a fluid or malleable state to a solid state. Thus, the extrusion process can be applied to materials which undergo a chemical reaction to form a solid cured product; to materials which dry either by the evaporation of water therefrom or by the absorption of water into a different crystal or morphological form; or to molten or thermoplastic materials which solidify on cooling. The extrusion process is of especial application in the extrusion of thermoplastic polymers such as polyalkylene resins, notably polyethylenes, polypropylenes and alloys or blends thereof. For convenience, the invention will be described hereinafter in terms of the extrusion of a polymer and the term extrudate will be used herein to denote in general all materials which can exist in a viscous, malleable, fluid or semi-fluid form which can be extruded under pressure through the orifice of a die and the term malleable will be used herein to denote the physical state of such a material as it is extruded.
Typically, the material to be extruded is fed as a particulate solid to a cylindrical tube known as the barrel of the extrusion apparatus and is fed along the barrel by a rotating screw drive or auger, a reciprocating ram or other positive transport means. If necessary, the barrel can be heated or cooled to maintain the material within the barrel at an optimum temperature for flow of material through the barrel. The material is forced through an orifice at the end of the barrel, typically an interchangeable die made from a tool steel or other wear resistant material and having an orifice whose cross sectional geometry substantially corresponds to that of the shape which is to be produced. Typically, the cross section of the bore which forms the flow path of the extrudate downstream of the barrel progressively changes until it has the shape of the die orifice. Such change preferably occurs smoothly and the terminal portion of bore may have a cross section which is substantially uniform and corresponds to that of the orifice located at the downstream end of the bore. For convenience, the term die will be used hereinafter to denote that portion of the extrusion equipment through which the extrudate flows downstream of the barrel; the term bore of the die will be used to denote the passage within the die through which the extrudate flows; and the term die orifice will be used to denote the orifice through which the extrudate flows from the downstream end of the bore. The die can be formed as a unitary member with the bore and orifice being machined or otherwise formed in a single metal component. Alternatively, the die can be formed in sections so that only the terminal section or sections need be replaced to enable the shape of the die orifice and hence of the product to be changed.
The extrusion of synthetic fibres is well known in the textile industry where spinnerets are used to extrude fibres prior to the fibres being woven into textiles. Such fibres are typically substantially cylindrical, although substantially triangular fibres are also known. To increase the insulating properties of the fibres and to reduce the amount of material used to form the fibres it is known for spinnerets to produce fibres having one or more bores therealong.
Single bore fibres have been formed using gas entrainment within the extrudate as shown in U.S. Pat. No. 3,397,427. This document discloses the use of a single gas outlet within the die orifice, the gas outlet being positioned centrally within the die orifice.
Fibres with multiple bores are also known in the textile industry, but these have been produced through the use of bluff voids as described in EP 0 317 192 and in U.S. Pat. No. 3,585,684 in which several extruded elements are caused to coalesce soon after exiting the die orifice. Such dies must be very carefully designed for a specific extrudate composition and volumetric flow rate as die swell (the swelling of extrudate as it exits the die orifice) may cause the desired bores within the material to vary in size, close up or to become unstable.
It is an object of the present invention to produce an extrudate having a plurality of hollow bores and to address some of the issues above.